The present invention relates to a Streptococcus thermophilus strain, to the use of this strain for the production of fermented dairy products and to the products obtained using this strain.
The choice of lactic acid bacteria for the production of fermented dairy products calls into play various criteria, especially the acidifying activity and the formation of flavor components which provide the organoleptic properties of the product and the production of thickening agents which play a role in the texture and the unctuousness.
The acidifying activity is characterized essentially by three parameters: the kinetic of acidification, the titratable acidity and the final fermentation pH which determines the organoleptic characteristics of the product and its preservation quality, and the postacidification which develops during preservation of the product.
A high rate of acidification makes it possible to reduce the period during which the milk-based product is sensitive to contaminants (pH greater than 4.7) and thereby to reduce the risk of bacterial contamination:
An increase in the rate of acidification also enhances the economics of the process by increasing the productivity and the flexibility of the industrial material.
The post-acidification properties of the strains are particularly important for the preservation or the products. Indeed, fresh fermented products are preserved at temperatures of between about 4xc2x0 C. and 8xc2x0 C. for a period in general not exceeding 4 weeks; but if the metabolic activity of the bacteria is reduced by preservation at low temperature, it is not blocked and it brings about the production of lactic acid from lactose, which has as a consequence a reduction in pH and an increase in the acid taste which damages the organoleptic properties of the product.
In addition to the criteria retained for their contribution to the quality of the products, other elements more specifically linked to the process play a role in the choice of the strains, such as the fermentation temperature, the rate of acidification and the resistance to phages.
The resistance to phages, is a very important criterion in the choice of the strains for reducing the risk of phage incidents during production which may block for a longer or shorter period the entire production for decontamination.
No S. thermophilus is so far known which exhibits a high rate of acidification and a high degree of acidity combined with a very low or practically zero post-acidification activity.
Indeed, it is generally observed that cultures of S. thermophilus which have a reduced post-acidification activity often show a limited acidifying activity during the fermentation, which does not allow the desired degree of acidity to be achieved in the products. In addition, these strains often have a slow growth and they require high inoculation levels and a longer incubation period which are incompatible with the economics of an industrial process.
The subject of the present invention is an S. thermophilus strain which has a rapid acidification kinetic allowing a high degree of acidity to be achieved and which does not post-acidify fresh fermented products during preservation.
More particularly, the present invention relates to an S. thermophilus strain DN-001 116 deposited at the Collection Nationale de Cultures de Microorganismes (CNCM) under No. I-1477 and mutants thereof which have similar milk acidification characteristics.
Mutant strains having milk acidifying characteristics similar to the deposited strain is intended to designate strains which may be obtained especially by mutation and selection from the reference strain and/or by genetic transformation with the aid of vectors.
The acidifying characteristics are an end-of-incubation pH of 4.6 in less than 4 hours and a pH variation at 28 days of less than 0.2 pH unit, preferably less than 0.1 unit.
The mutant strains may be obtained from the reference strain, especially by mutation and selection according to their acidifying properties. The mutation techniques are known, as well as the tests which make it possible to control the acidifying properties (especially Spinnier H. E., Corrieu G. xe2x80x9cAutomatic method to quantify starter activity based on pH measurementxe2x80x9d J. of Dairy Research, 56 (1989) 755-764); by Roissart H. and Luquet F. M. Edition Lorica ISBN: 2-9507477-0-1).
These strains are more particularly advantageous for the preparation of all types of fermented milk products.
Accordingly, the present invention relates to a process for the preparation of fermented dairy products in which a milk substrate is fermented with at least one S. thermophilus strain.
It is also possible to provide, in this process, for the use of a combination of one or more bacterial strains, especially with other lactic acid bacteria such as Lactobacillus bulgaricus, lactobacillus aciddphilus, Lactobacillus casei or Bifidobacterium.
Preferably, the milk substrate is natural or reconstituted milk, skimmed or otherwise, or milk-based media or media based on products of dairy origin.
This substrate may comprise items commonly used for the preparation of milk desserts, solid items such as fruits, chocolate chips or cereals for example, but also sweetened products or liquid chocolates. The strain according to the invention in fact has the advantage of being resistant to sugars, which makes it possible to ferment sweetened products containing especially sucrose.
The present invention also relates to the fermented milk products obtained using this process, especially fresh fermented cheese, and yoghurts for example.
The products obtained using this strain have a mild taste and their organoleptic properties are preserved during storage.
The culture of the strains according to the invention, pure or in combination with other strains, may also be used as probiotic for human consumption or animal feed and also as ferment to be used in the process according to the invention.